Silicon carbide (SiC) is one of compound semiconductors and is thermally and chemically stable. Compared to silicon (Si), SiC has advantageous physical properties such as a band gap which is approximately three times as large, a dielectric breakdown voltage which is approximately ten times as large, an electron saturation speed which is approximately twice as large, and a coefficient of thermal conductivity which is approximately three times as large. On account of these excellent properties, SiC has attracted interest as a material for the next generation of low-loss power devices. In order to manufacture such a device, it is necessary to provide a SiC single crystal wafer with a SiC epitaxial film which is formed by epitaxial growth of a SiC film serving as an active layer of the device on a SiC single crystal substrate. For practical applications, the wafer diameter needs to be at least 2 inches.
SiC is well known as a substance having crystal polymorphism called polytypes. This crystal polymorphism is a phenomenon in which a substance can take a plurality of crystal structures which differ only in the manner of stacking of atoms in the c axis direction while maintaining stoichiometrically the same composition. Typical polytypes of SiC are 6H (a hexagonal crystal having 6 molecules per period), 4H (a hexagonal crystal having 4 molecules per period), 3C (a cubic crystal having 3 molecules per period), and the like. 4H—SiC is said to be particularly preferred for power devices.
In both substrates and epitaxial films, coexistence of two or more polytypes has an adverse effect on device performance. Therefore, good quality SiC single crystals having a pure crystal form (without forming a mixture of different polytypes) and minimized crystal defects are necessary.
Known methods for manufacturing SiC bulk single crystals for use as a SiC single crystal substrate include the sublimation recrystallization method and the solution growth method. Known methods of growing a SiC epitaxial film include the chemical vapor deposition (CVD) method and the liquid phase epitaxy (LPE) method.
Almost all epitaxial film-coated SiC single crystal wafers which are currently commercially available have a substrate portion manufactured by the sublimation recrystallization method and an epitaxial film portion formed by the CVD method (more particularly, by the below-described step-controlled epitaxy technique). Namely, each portion is manufactured using crystal growth from a vapor phase.
In the sublimation recrystallization method which is a manufacturing method for bulk single crystals, a raw material SiC powder is sublimated at a high temperature of 2200-2500° C., and a SiC single crystal is recrystallized on a seed crystal made of a SiC single crystal disposed in a lower temperature region. This method has a high growth rate. However, the SiC single crystal which was grown contains a large number of dislocations and micropipe defects which are propagated from the seed crystal as well as a large number of dislocations which are thought to generate during crystal growth. Accordingly, in the sublimation recrystallization method, it is extremely difficult to obtain a SiC single crystal having a quality significantly superior to that of the seed crystal.
When a SiC bulk single crystal is grown by the solution growth method, carbon is dissolved in a melt of Si or a Si alloy serving as a solvent to prepare a solution of SiC dissolved in the melt. A SiC seed crystal is immersed in this SiC solution, and a state of supersaturation of SiC is formed by supercooling the solution at least in the vicinity of the seed crystal to grow a SiC single crystal on the seed crystal. In order to create the supersaturated state, the so-called temperature difference method in which a temperature gradient is provided so that the temperature of the melt in the vicinity of the seed crystal is lower than that in other areas is typically used. Compared to the sublimation recrystallization method, the solution growth method which employs liquid phase growth can lower the growth temperature by around 500-1000° C.
In the CVD method which is a vapor phase growth method for an epitaxial film, a mixture of a silane gas and a hydrocarbon gas which are raw material gases are thermally decomposed, and a SiC film is deposited on a substrate. This method has the drawback that during the process of forming a SiC single crystal film, two or more polytypes are intermixed in the resulting film. In order to overcome this problem, the step-controlled epitaxy technique in which a substrate having its surface sloped by a few degrees in the (112-0) direction from the c axis (an off-axis or off-oriented substrate) is used to perform the growth in the step-flow mode (lateral growth) of an epitaxial film of the same polytype as the substrate has been proposed.
However, in the step-controlled epitaxy technique using an off-axis substrate, dislocations in the substrate are propagated to the resulting SiC epitaxial film and it is difficult to obtain an epitaxial film having a decreased number of dislocations. In addition, SiC epitaxial films formed by the CVD method in general may include lattice defects such as vacancies or interstitial atoms. With such a film, it is not possible to manufacture a semiconductor device having good properties with respect to breakdown voltage, leak current, and the like.
In the liquid phase epitaxy (LPE) method which forms an epitaxial film in a liquid phase, a SiC single crystal film is formed on a substrate by nearly the same principles as for the solution growth method which grows a bulk single crystal in a liquid phase. Namely, using a SiC solution having a melt of Si or a Si alloy as a solvent, a SiC epitaxial film is grown on a substrate under supersaturation of SiC. In contrast to the CVD method, the LPE method performs crystal growth in a state close to thermodynamic equilibrium, which makes it possible to obtain a SiC epitaxial film having a low density of crystal defects.
Below-identified Non-Patent Document 1 reports that growth of a SiC single crystal by the LPE method on an on-axis (0001) SiC single crystal substrate prepared by the sublimation recrystallization method causes crystal growth to proceed while decreasing micropipe defects and dislocations. It is speculated that a SiC epitaxial film having improved crystal quality can be formed by that method.
However, although the LPE methods which have been proposed thus far could decrease crystal defects in a SiC epitaxial film, it was difficult to obtain a SiC epitaxial film having a low doping concentration suitable for power devices (specifically, a doping concentration of nitrogen donors of 1×1016/cm3 or lower).
In SiC epitaxial growth by the LPE method described in below-identified Non-Patent Document 2, when the growth atmosphere was helium, argon, or a vacuum (5×10−4 Pa), the doping concentration of nitrogen donors as background impurities in the resulting SiC epitaxial film was approximately 3×1018/cm3, approximately 1×1017/cm3, or 2×1016/cm3, respectively. This nitrogen concentration is thought to be due to nitrogen gas present in impurity gas components which remain in the atmosphere dissolving in the SiC solution, and dissolved nitrogen atoms being incorporated into the grown crystals as n-type donor impurities. The above-described concentrations of background impurities are high for an epitaxial film for power devices. That document also states that in crystal growth in a vacuum in which the lowest doping concentration of nitrogen donors was achieved, there is violent vaporization of the melt and stable crystal growth is not possible.
Below-identified Non-Patent Document 3 states that a SiC crystal with a doping concentration of 8×1015/cm3 can be obtained by the LPE method using Si as a solvent under a vacuum on the order of 5×10−5 torr, i.e., approximately 6.67×10−4 Pa. In the LPE method used in that document, a Si melt is not housed in a crucible, and it is upthrusted by electromagnetic force produced by a water-cooled induction coil. After C is dissolved in the melt, a SiC crystal is grown on an extremely small SiC substrate measuring 0.5-1.5 cm2. Crystal growth is performed in a vacuum, so as stated above for Non-Patent Document 2, there is a concern of vaporization of the melt. Furthermore, that method has the problem that it is difficult to carry out crystal growth on a SiC substrate having a large area for practical applications. In order to make it possible to use a substrate having a large area, it is necessary to increase the amount of melt which is upthrusted, and for this purpose, a high frequency oscillator of extremely large electric power becomes necessary.